My english is not the best, but I will try explaining it to you!
first of all you need Mastix www.ebay.de/itm/Grimas-Mastix-... I don't know if you have an ebay account, but I think you can buy it at carneval/costume stores too. It's actually just skin glue.
I bought on purpose a long wig, because when I cut it to the right lengh I had much hair left over and that is what I need for the beard! you just take the left over strands of hair and cut really small stubbles off, like 1-2 mm, just as small as possible! after that you maybe line the shape of leonardos beard with an skin-colored kajal or something.
Then just use the mastix at the spots and place the stubbles there.
Thank you so much! I thought about doing that but I didn't know that skin glue actually existed... I tried using watercolors, the effect was nice but it was so hot that the beard kept fading away. I don't know what I would've done without your tip for the next convention! Also my English is very bad but I'm Italian, I'm sorry >_> and yours is far better than mine hehe c: